Nickel ethylene polymerization catalysts based on phosphorus ligands

Article abstract of DOI:10.1021/om010705p

Nickel(II) complexes of ligands of the type Ar₂PN(Me)PAr₂ (Ar = ortho-substituted phenyl group) are highly active and poison-tolerant catalysts for the polymerization of ethylene.

Full text of DOI:10.1021/om010705p

Organometallics 2001, 20, 4769-4771
4769
Nick el Eth ylen e P olym er iza tion Ca ta lysts Ba sed on
P h osp h or u s Liga n d s
Neil A. Cooley, Simon M. Green, and Duncan F. Wass*
BP, Chemicals Stream, Chertsey Road, Sunbury-on-Thames,
Middlesex, TW16 7LL, United Kingdom
Katie Heslop, A. Guy Orpen, and Paul G. Pringle
School of Chemistry, University of Bristol, Bristol, BS8 1TS, United Kingdom
Received August 6, 2001
Summary: Nickel(II) complexes of ligands of the type Ar2-
PN(Me)PAr2 (Ar ) ortho-substituted phenyl group) are
highly active and poison-tolerant catalysts for the po-
lymerization of ethylene.
The discovery of high-activity nickel and palladium
olefin polymerization catalysts by Brookhart and co-
workers in 1995 has led to an upsurge of interest in the
use of late transition metal polymerization technolo-
gies.1 We report here our discovery of a new class of
ethylene polymerization catalyst based on nickel(II)
complexes of bulky bis(diarylphosphino)methylamines.2,3
The growing family of active nickel catalysts is almost
exclusively based on hard nitrogen or oxygen donors,4,5
and despite the central role of diphosphine ligands in
many other types of homogeneous catalysis involving
late transition metals,6,7 softer phosphorus donors have
been largely unsuccessful in this application.8
Several activation methods have been employed (eq
1). Initial polymerization experiments utilized a method
developed for the rapid screening of potential nickel
catalysts.4b,9 Treatment of [Ni(COD)2] (COD ) 1,5-
cyclooctadiene) with the ligand and H(Et2O)2BAF (BAF
) B[3,5-(CF3)2C6H3]4) in toluene and exposure of these
solutions to ethylene gave polyethylene with moderate
activity (Table 1).
Compound 1a , bearing isopropyl substituents, pro-
duces high molecular weight material with low levels
of branching consistent with “chain walking” mechan-
isms.4b Ligands 1b and 1c, with less sterically encum-
bered substitution patterns, yield lower molecular weight
material. The tolerance of the systems to traditional
poisons for olefin polymerization catalysts matches the
most robust existing nickel systems;10 with water levels
as high as 10% by volume, polymerization is still
observed, albeit at a tenth the activity.
To investigate a range of other activation methods, a
dibromonickel(II) procatalyst complex 2a was formed by
treatment of [NiBr2(dme)] (dme ) 1,2-dimethoxyethane)
with ligand 1a in dichloromethane according to eq 1. A
single-crystal X-ray structure of complex 2a was ob-
tained (Figure 1).11 The complex is C2 symmetric with
square planar coordination at nickel and a near planar
(1) (a) J ohnson, L. K.; Killian, C. M.; Brookhart, M. J . Am. Chem.
Soc. 1995, 117, 6414. (b) Gates, D. P.; Svejda, S. A.; On˜ate, E.; Killian,
C. M.; J ohnson, L. K.; White, P. S.; Brookhart, M. Macromolecules
2000, 33, 2320. (c) J ohnson, J . K.; Killian, C. M.; Arthur, S. D.;
Feldman, J .; McCord, E. F.; McLain, S. J .; Kreutzer, K. A.; Bennett,
A. M. A.; Coughlin, E. B.; Ittel, S. D.; Parthasarathy, A.; Tempel, D.
J .; Brookhart, M. WO 96/23010 (to DuPont) 1996 [Chem. Abstr. 1996,
125, 222773t].
(2) Wass, D. F. WO 01/10876 (to BP Chemicals).
(3) We have recently reported the activity of these ligands in
palladium-catalyzed ethylene/CO copolymerization: (a) Dossett, S. J .
WO 97/37765 (to BP Chemicals). (b) Dossett, S. J . WO 00/06299 (to
BP Chemicals). (c) Dossett, S. J .; Gillon, A.; Orpen, A. G.; Fleming, J .
S.; Pringle, P. G.; Wass, D. F.; J ones, M. D. Chem. Commun. 2001,
699.
(4) (a) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 429. (b) Ittel, S. D.; J ohnson, L. K.; Brookhart, M.
Chem. Rev. 2000, 100, 1169.
(5) Mixed hard/soft ligands produce polyethylene under certain
conditions: (a) Keim, W.; Kowaldt, F. H.; Goddard, R.; Kruger, C.
Angew. Chem., Int. Ed. Engl. 1978, 17, 466. (b) Keim, W.; Appel, R.;
Gruppe, S.; Knoch, F. Angew. Chem., Int. Ed. Engl. 1987, 26, 1012. (c)
Klabunde, U.; Ittel, S. D. J . Mol. Catal. 1987, 41, 123. (d) Klabunde,
U.; Mulhaupt, R.; Herskovitz, T.; J anowicz, A. H.; Calabrese, J .; Ittel,
S. D. J . Polym. Sci., Part A: Polym. Chem. 1987, 25, 1989.
(6) Notably, palladium phosphine catalysts are well established for
the copolymerisation of olefins and carbon monoxide. See: Drent, E.;
Budzelaar, P. H. M. Chem. Rev. 1996, 96, 663.
(7) For a recent general overview of the use of phosphine ligands in
catalysis, see: van Leeuwen, P. W. N. M.; Kamer, P. C. J .; Reek, J . N.
H.; Diekes, P. Chem. Rev. 2000, 100, 2741.
(8) Chelating diphosphine ligands demonstrate extremely low activ-
ity and give low molecular weight products: (a) Mui, H. D.; Riehl, M.
E.; Wilson, S. R.; Girolami, G. S. ACS Abstracts 1994, Vol. 208 (part
1), pp 530-INOR. (c) Hoehn, A.; Lippert, F.; Schauss, E. (BASF) WO
96/37522. (e) Brookhart, M. S.; Feldman, J .; Hauptman, E.; McCord,
E. F. (DuPont) WO 98/47934.
(9) J ohnson, L. K.; Feldman, J .; Kreutzer, K. A. (DuPont) WO
9702298.
(10) (a) Held, A.; Bauers, F. M.; Mecking, S. Chem. Commun. 2000,
301. (b) Younkin, T. R.; Connor, E. F.; Henderson, J . I.; Friedrich, S.
K.; Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460.
10.1021/om010705p CCC: $20.00 © 2001 American Chemical Society
Publication on Web 10/13/2001

4770 Organometallics, Vol. 20, No. 23, 2001
Ta ble 1. Eth ylen e P olym er iza tion Resu lts
Communications
branchesc/
1000 C
entry
catalyst
methoda
T (°C)
p (bar)
yield (g)
activityb
Mw
Mn
PDI
1
2
1a
1b
1c
1a
2a
2a
2a
2a
2a
1a
1b
1c
2a
i
i
i
i
25
25
25
25
25
25
50
50
50
50
50
50
50
1
1
1
1
1
8
8
20
8
8
1.0
0.8
0.6
0.1
4.1
15
100
80
60
120 000
23 000
9000
85 000
98 000
148 000
65 000
78 000
62 000
73 000
32 000
9000
4000
3.7
2.5
2.2
2.5
2.4
3.7
3.8
5.9
2.8
3.0
2.7
11
3
8.0
2.5
3.1
1.6
2.3
3.1
6.2
2.5
22
4d
5
10
34 000
40 000
40 000
17 000
13 000
22 000
24 000
42 000h
11 000h
21 000
iiie
iii
iii
iii
ivf
vg
vg
vg
ivg
820
1500
2200
1500
210
225
24
6
7
8
9
10
12
13
14i
22
15
4.2
11
1.5
0.4
1.0
8
8
8
7
17
36
63 000
3.0
11j
a
b
All runs using 0.01 mmol catalyst and 1 h run time unless otherwise stated. Average activity in g/mmol‚h over run time; some
catalyst deactivation is observed over this time (see kinetic profile included in Supporting Information) and initial activities are higher.
c Methyl branches predominate, although smaller amounts of ethyl and longer branches are observed. Typical data included in Supporting
Information. Toluene solvent contains 10% v/v water. e Run terminated after 30 min; after this time the polymer produced hindered
d
stirring. f 0.02 mmol catalyst used. 0.04 mmol catalyst used. Mpeak: Bimodal distribution obtained with additional low molecular
g
h
i
j
peak at 300. 5 ml 1-hexene added. Butyl branches; 4.2 methyl branches also present.
used;13 active catalyst systems are formed only with
cocatalyst batches containing no free trimethyl alumi-
num.14 Furthermore, addition of trimethyl aluminum
to an ongoing polymerization inhibits further activity.15
Using a MAO activation procedure results in higher
activity compared to the [Ni(COD)2]/H(Et2O)2BAF method
(compare entries 1 and 5), although the structure of the
polyethylene obtained is very similar.
The effects of temperature and pressure have been
investigated. As temperature is increased from 25 °C
to 50 °C, the rate of polymerization increased and the
molecular weight of the resultant polymer decreased.
The catalysts show some deactivation with time;16 initial
activity for entry 7, measured over the first 10 min of
the polymerization run, is as high as 6000 g mmol-1
h-1. Polymerization rate and molecular weight have a
zeroth-order dependence on ethylene pressure (entries
7 and 8), as observed with other nickel polymerization
catalysts.1
F igu r e 1. Molecular structure and numbering scheme for
2a . All but the tertiary isopropyl hydrogen atoms have been
omitted for clarity. Important molecular dimensions: bond
lengths (Å) Ni(1)-P(1) 2.1611(6), Ni(1)-Br(1) 2.3276(4);
bond angles (deg) P(1)-Ni(1)-P(1a) 74.07(3), P(1)-N(1)-
P(1a) 92.49(7).
Catalyst systems may be formed using a MAO-treated
silica cocatalyst (eq 1, method iv).17 This procedure
provides a method for generating MAO-based cocata-
lysts that are free of trimethyl aluminum. Supported
catalysts may also be conveniently obtained by mixing
[Ni(acac)2] (acac ) acetylacetonate), ligand, and MAO-
silica in situ (eq 1, method v); the results obtained with
either method are the same (compare entries 9 and 10).
As is usual, the supported catalysts investigated show
somewhat lower activity compared to unsupported
analogues (entries 7 and 9), although it is interesting
to note that the product polymer properties remain
largely unchanged. Reducing the steric bulk of the
ligand again leads to a reduction in polymer molecular
weight and a considerable reduction in the activity
observed. Introduction of 1-hexene comonomer to a
NiP2N chelate ring. As a consequence, the aryl groups
are isoclinal, and two have ortho isopropyl groups
orientated so as to block the axial sites at nickel. The
conformation of the other aryls (at C10 and C10A) is
such as to provide some blocking of the freedom of
ligands (or polymer chain) bound in the equatorial
coordination plane of the nickel. Steric bulk is important
in reducing chain transfer rates, as described for other
nickel polymerization catalysts.1 In this case, we propose
the sterically encumbered chelates may also prevent the
formation of dimeric nickel species, often observed with
one-atom backbone diphosphines.12
Treatment of 2a with MAO (methyl alumoxane)
cocatalyst yielded a highly active polymerization cata-
lyst. However, the reactivity observed is very dependent
on the free trimethyl aluminum content of the MAO
(13) Similar effects have been observed for metallocene-type cata-
lysts. For a discussion of this and a recent review of MAO cocatalysts
in general, see: Chen, E. Y.-X.; Marks, T. J . Chem Rev. 2000, 100,
1391.
(14) Addition of an excess of Lewis base, in this case 1,4-dioxane,
to MAO solutions allows the free trimethyl aluminium content to be
quantified using 1H NMR spectroscopy (see Supporting Information).
(15) See Supporting Information.
(11) 2a : C37H47Br2NNiP2, M ) 786.23, monoclinic, space group C2/c
(no. 15), a ) 16.6700(17) Å, b ) 8.9444(10) Å, c ) 24.614(4) Å, â )
102.024(8)°, U ) 3589.4(8) Å3, Z ) 4, µ ) 2.882 mm-1, T ) 173 K,
4119 unique data, R1 ) 0.0280. Molecules of 2a have exact crystal-
lographic C2 symmetry.
(16) A typical profile is included in the Supporting Information.
(17) For
a recent review of support strategies, mainly in the
(12) Puddephatt, R. J . Chem. Soc. Rev. 1983, 12, 99.
metallocene area, see: Hlatky, G. G. Chem. Rev. 2000, 100, 1347.

Communications
Organometallics, Vol. 20, No. 23, 2001 4771
polymerization leads to incorporation of this higher
1-olefin (entry 14), as shown by an increase in polymer
butyl branches.
Ack n ow led gm en t. The authors thank J ames Flem-
ing, J ames Dennett, and Roy Partington. Graham
Barlow and J ane Boyle are thanked for NMR analysis.
Stephen Dossett, Peter Maddox, and Glenn Sunley are
thanked for useful discussions.
In conclusion, these new nickel compounds are highly
active and poison-tolerant catalysts for the polymeri-
zation of ethylene. Catalyst performance approaches
that of the best existing nickel-based systems.1,10b Given
the great array of related chelates that are synthetically
accessible and widely used in other areas of catalysis,
it seems likely that yet more active and selective
phosphorus ligands for this application await discovery.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails. Typical polymerization gas-uptake profile. Crystal-
lographic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM010705P